You should not feel threatened by other men. Women go places with friends (male or female) for many reasons. Do not let it phase you. You can even spend a bit of time getting to know this "neighbour" of hers. If he is a legit friend and you can win him over then you can win her. Personally, I would have given the friend the attention to figure out where he stands and go from there. Here is a little clip of something that happened to me:
I had arranged to meet up with this girl at a club. When she arrived she was with a male friend and introduced him as such. I greeted both of them politely and then started talking to the guy to figure out what his interests were, how him and this girl met, etc. I found out the he was single, straight, and quite entertaining.We took to the dance floor and the rest is a fond memory.
He and I started talking about which girls in the club we thought were hot (right in front of the girl) and all of a sudden she had the look of betrayal on her face. She obviously felt like her friend betrayed her and I thought it was the funniest thing ever. She even would interject with comments like:
"How can you like that girl... she's got *blah* *blah*"
After hearing these comments 2 or 3 times the following response left my lips:
"I'm entitled to my opinion, thank you; however, would you like to prove that you can dance better than her too?"